Annual leave: Cornflowers, love-in-a-mist, Nigellas... not only have they got great names, annuals cheer up September too

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Annuals have kept the garden going through September, not substantial fare, but enough to provide some wandering strands of colour between the mounds of euphorbia foliage and the strappy grey swords of iris. It seemed a disaster, early this year, when my first sowings of ammi got munched by slugs. Seedlings from the subsequent sowing took time to settle in the June heat (there was a bit of summer at that stage) and did not start flowering properly until early August. The damp cool weather we had then must have suited them for they developed into beautiful plants.

Annuals rarely have good foliage but ammi (Thompson & Morgan £1.99) is an exception, producing a foamy mass of dull green, very finely cut. You get bulk, but bulk with finesse. Over this appears wave after wave of flat white flower heads, 11-12cm across, made up, like cow parsley, of masses of individual flowers. Insects love them. Each stem produces 30 or more heads and they take it in turns to perform. That means the show goes on a long time.

Next year (gardeners can never resist looking ahead – next year is always going to be better than this one) I'm hoping to have two waves of ammi. The seed for the first lot was sown at the beginning of this month, scattered on multipurpose compost in a 12cm pot. They germinate quickly. I have pricked out the seedlings into 8cm pots. It's laborious work, but you get good, stout plants this way. If I'm more vigilant about slugs than I have been this year, I should have fine plants to set out in April and these will be in flower in June. Then next year, I will sow another batch in May, deliberately late, to provide the flowers I have so enjoyed these past two months: ammi collapsing into the woody stems of a spice-flavoured pelargonium, ammi mixing with the second flowering of giant chives, ammi with cornflowers, equally happy with the more purplish blue of stokesia, ammi with the now dulled heads of cerinthe.

Altogether too much ammi you might feel. But it's an easy, casual guest. Wherever it goes, it is welcome. Unstaked, it flops on to its elbows. But sometimes this is an advantage, as it is along the stone path that climbs our bank. There, it threads itself through chives, pinks, violas, late alliums and, because it is so airy, never smothers them. The ammi I've been talking about is Ammi majus which comes from the areas round the Mediterranean: southern Europe, Turkey, North Africa. Next year, I'm going to try Ammi visnaga as well, similar in height to Ammi majus but with more rounded, slightly denser heads, a Fibonacci marvel.

Cornflowers, for me, must be blue. The purplish ones, like 'Black Ball', may be fashionable, but nothing else in the garden gives quite such a searingly fantastic blue as 'Blue Boy' or 'Blue Diadem' (Thompson & Morgan £2.19). Stay away from dwarf forms. There's some weird thinking among flower breeders that small gardens need dwarf plants. The opposite is true. You need plants that provide maximum impact. Perhaps there'll be fewer of them than you would find in a bigger garden, but you need to be even surer that they are earning their keep. A decent cornflower should stand at about 75cm (30in).

I sowed seed in late March and again pricked out the seedlings into individual pots before planting in May when the early bulbs had gone over. The main flush has busted, but there are still, in late September, a few unmissable sparks of blue among the last pale lemon daisy flowers of Anthemis 'E C Buxton'.

Cornflowers are among the annuals that you can sow direct into the garden, but this is a chancier business. It works best in places where there is plenty of room for the seedlings to develop. Since I mostly want cornflowers to interplant with other things on the bank, raising decent plants in pots works best for me.

But I do direct sow other annuals – Californian poppies, English marigolds, love-in-a-mist – mostly in the gravelled areas where I grow spring bulbs. Since the bulbs need all the baking they can get in summer, I don't want anything too hefty sitting on their heads. Sprinkled now, or in spring between the dying flowers of the bulbs, each of these three annuals performs well. There's now a self-sustaining colony of love-in-a mist over the big gravelled triangle where species crocus, Iris bucharica and some tricksy wild tulips grow.

Here, I started with two kinds, sprinkled on to the gravel: Nigella damascena 'Miss Jekyll' (Chiltern Seeds £1.37) which gives the classic love-in-a-mist with sky-blue flowers and Nigella hispanica (Chiltern Seeds £1.97) which has much darker flowers, almost navy. Both have excellent seed pods, one rounded, the other urn-shaped with strange horns curling out of the top.

The plants shed their seed from early August onwards and these self-seeded plants are of course the earliest into flower the following season. Each spring, I add fresh seed of one or other kind to give another later flush.

Early this spring, I noticed that two extraordinarily strong plants had pushed up through an impossible crack by the stone retaining wall. Self-sown of course. But they developed in an unusual way. Nigella is often quite spindly, but these made stout, broad plants with 14 separate stems. The better of the two carried 62 flowers. I counted, because last month I cut off all the seedheads to collect the seed.

The two plants intrigued me for another reason too. When the flowers first came out, they were sky-blue like Nigella damascena. But they darkened as they matured, so they finished the midnight-blue of Nigella hispanica. The seedpods are rounded, like those of Nigella damascena. My guess is that they are a chance cross between the two species, a mongrel with hybrid vigour. I doubt the seeds will produce a mass of plants like the parent. But it will be interesting to see how much the plants vary. There's plenty of seed, so I've sown some now and will do further batches next spring. If you like the idea of a mixed batch of flowers, try Nigella damascena 'Moody Blues' (Thompson & Morgan £1.99).

One way and another, I've enjoyed the garden this month. It has the end-of-season scruffiness that is inevitable if you hang on to self-seeding plants, as I do. The paths are sprouting a new crop of weeds (mostly forget-me-nots), the stems of Deschampsia cespitosa have collapsed all over their neighbours, as they always do, the courgettes are covered in mildew. But the agapanthus have been outstanding and the lilies have given the best show I've ever had. And I made a plum tart with the first 'Landkey Yellow' plums ever to fruit in our new orchard.

'Bulb' by Anna Pavord is published by Mitchell Beazley, £30. To order a copy at a special price, including p & p, call Independent Books Direct on 08700 798 897